Last week we had another of our training courses for service engineers and field service engineers1. The interaction with people actually working with our products is quite refreshing and every time I understand their issues better. One of those issues is that they have to discuss with the client are spares for the wear parts. In a planned maintenance context, wear parts are a little odd. Sometimes, they are worn away or break down unexpectedly. And that is the moment customers call for spares. We do have a lot of spares on stock, but sometimes even we run out of stock or we advise to use a special execution of the concerned part for the specific operation of the client. And then we have to inform the service people and the client that there is a long lead time. Several times, they are filled with disbelief and under such circumstances it is very difficult to explain the reasons behind it. So, that is why I developed this little game to experience the waiting time for special wear parts.
It is based on the old board game of ‘Snakes and Ladders’2. All it takes are the board, one dice and as much tokens as players. All start at the first position. The places are all phases in the manufacturing of the wear parts and each have their specific issues.
Each pattern is used multiple times and wears down, itself. Also, some patterns have to be configured for the specific application, execution or material of the wear part.
Moulding: the pattern is placed in a casting box and filled with sand.
Sometimes there are more casting boxes needed and they have to be stacked carefully. Depending on the configuration, this step might be very short. Then you go directly from 2 to 4.
Part of the casting system is already in the sand box with the pattern, but it has to be finished as the last part of the preparation.
The material is melted in the furnace. This can take some time, depending on the size of the cast.
The actual casting is done in minutes. Fifteen, at the most.
But the cooling in the casting box takes weeks. Wait one turn.
Sometimes the casting has not gone properly and the cast have to be done again. Back to square one.
Satisfied with the cast, then it has to be touched up at the fettling station.
A special heat treatment brings the final hardness and toughness to the product.
The fitting surfaces of the wear parts have to be machined.
Rotating parts have to be balanced. For non-rotating parts, this can be skipped.
Then there is the bottle neck: quality control. If there is a deviation that can n ot be mitigated, you have to go back to square one.
Depending on the location, transport can take weeks.
Don’t start me about customs handling. Your anticipated spares are in bonded storage and customs is missing a document, wait some weeks or skip a turn.
Finally, you’ve made it! Installation on the dredge.
Message of the game: keep your warehouse well stocked with wear parts3,4, or your dredge will be idle for months, before you can work again. Have fun!
Yesterday, Suman Sapkota graduated at the University of Twente on: ‘Technical and Sustainability Analysis of Sediment Erosion of Impeller Blades of Dredge Pumps’. We are very proud he worked for us on this topic and graduated with a grade 8 for it. As such, he stands in a long line of Master students1, who graduate with an 8 or even higher2. We value good students and we like to work with them only if the work they deliver is useful for us. This requires an intensive supervision and the results are correspondingly. This does not guarantee a good result, but you can always try3.
Suman graduated on a topic covered by the chair of ‘Sustainable Energy Technology’4. The objective was to study the economic and environmental impact of worn wear parts. When wear parts have to be rejected too early, they increase their environmental footprint. On the other end, if an impeller is severely worn, the efficiency decreases and the environmental footprint increases also. Our question was whether it was possible to improve the design of the dredge pump for a longer lifetime by checking the wear rate of the improvement in a simulation. To understand the problem and answer the question Suman had to start investigating the wear process itself.
From literature he evaluated different erosion models. Basically, what is the effect when a defined particle impacts the concerned surface under certain conditions. And the conditions will be depending on the flow pattern and the influence of the fluid on the particle trajectories. The second research part was the investigation of this flow pattern and simulated with a CFD application. The flow pattern and the particle properties were combined. This way, the movement of the particles can be traced from inlet to outlet.
Particle tracking in CFD simulation of an impeller
Between inlet and outlet, the particles bounce against several areas of the blade. The bouncing can be counted and mapped. Together with the information of the impingement conditions and the erosion model, an estimation of the erosion rate can be made for different flow conditions and soil properties. The mapping can be transformed into an erosion density map. These maps can be checked for pump capacity and particle sizes.
With these maps, we can finally evaluate in advance where impellers will wear down and if we can improve their lifetime by modifying the design. Thus we can reduce their environmental footprint and contribute to a more sustainable business.
Suman, thank you very much for working for us. You were a pleasure to work with. We wish you good luck on your career and a happy life.
Summer is arriving. We’ve had lots of sunshine and thunderstorms with rain these days. For sunshine, you protect yourself with lots of sun screen with a high protection factor. If it rains, you put on a raincoat and wellies. So, if you want to protect what is dear to you, you cover it with the appropriate cover. Hmm, if your pump gets eroded by your mixture, you cover it with a protective layer. Right?
So, let’s see what options we have? Common solutions to protect the wear parts of the pump are:
Vulcanise a rubber film over a new cast wear part. Usually, the pump parts are designed to receive an additional layer of several mm to a couple of cm. Astoundingly, the soft rubber, lasts longer than the hard alloy. This is due to the elasticity of the rubber. Impacting particles are bouncing back into the fluid and don’t damage the metal1. There have been several developments, where polyurethane2 can be a viable alternative to rubber with the same protective principle. Rubber and polyurethane have to be applied by specialised companies under controlled conditions. One warning though, rubber can be cut. When dredging shells or coral, the rubber is sliced to pieces and the flow peels away sheets of rubber or PU.
Instead of the flexible rubber, also hard chemical compounds have been developed to be applied as protective layer. This layer can be applied on a virgin part, that can accommodate the layer by design, just like the rubber cover. Or it can be used to restore an already worn down part and extend the lifetime that way. The pastes have been engineered to be applied in the field: wet, rust, salt and dirt. As it easily applied, many owners are very fond of this solution. Although lifetime is not extended as much as the first option.
Another process for new builds and restoration is hard-facing3. On a suitable base alloy, the vulnerable surface is cladded by welding a layer of very hard metal onto it. The hard-facing can be much harder than the sand particles (Mohs remember) and that the added material is brittle as glass doesn’t matter as it is carried by a much more ductile base material. As the base material is also softer, once the hard layer is away, the wear rate will become unexpectedly rapid. Especially as such local wear spots tend to eat through the material, due to the increased turbulence. Special care should be taken, when the wear part is not completely covered in hard-facing, but only partially covered. The discontinuities are hot spots of wear. Discontinuities can also appear from the hard-facing itself. The different material properties of cladding and base material cause to be vulnerable to check cracking and flaking4.
All three solutions are labour intensive. especially vulcanising and hard-facing takes many hours. And applying the wear paste has to be done more often. Even if labour is cheap in your operation, you still have to take it into account in your spare part strategy. More so, if you only rely on just one wear part and don’t have one on stock and the dredge is idle, you’ll lose a lot of money on income.
Another consideration might be the environment. An all metal wear part can be recycled easily. Rubber, PU and paste can be a pain to get rid of, responsibly.
Off course, I am an engineer at the manufacturing side and the above perspective may be biased, but I like to be proven wrong. Until then, I would rather purchase a durable all metal wear part, than go to the trouble and costs of the extra handling. Whatever your final wear part strategy, it should revolve around having the correct spare parts at the right time at the right location.